Much of pulmonary physiology is based on the analysis of respiratory gases. Because of its potential as a high speed accurate gas analyzer, the mass spectrometer has attracted considerable attention in this field. However, the instrument has failed to reach its potential at least in part due to the necessity for a long capillary inlet system which can, and often does, destroy the integrity of the gas sample and causes instability in the instrument.
Thus, while mass spectrometers have been available to respiratory physiologists for about 20 years, they have not achieved the widespread application that was once predicted. With respect to the technical shortcomings in spectrometer design at least for pulmonary physiology purposes, the sample inlet system is one of the major problems.
Inherent with mass spectrometry as well as with a sputtering system is that an immense pressure difference exists between the site at which gas is sampled and the inside of the spectrometer. Traditionally this pressure drop is achieved in two stages. Firstly, a long slender sampling capillary tube is used which produces the major fall in pressure. Secondly, at the end of the capillary a fixed molecular leak is employed to achieve the final pressure drop.
The capillary is required because the size of known spectrometers does not permit them to be brought into close proximity to the source of sample gas such as a respiratory valve. (See Fowler, K. T., "The Respiratory Mass Spectrometer," PHYSICS IN MEDICINE AND BIOLOGY, Volume 14, pages 185-199, 1969.) This arrangement has several adverse effects on instrument performance. Firstly, there are distortions introduced by the behavior of water vapor. During the respiratory cycle sample gas swings between dry inspired and wet expired gas. Water vapor traverses a heated sampling capillary about 10 times more slowly than the other respiratory gases. Hence, the ionizer sees a fluctuating water vapor level which does not reflect the pressure of water vapor at the front end of the capillary. Unpredictable errors in precision occur because the dilution effect due to water vapor is not the same as existed at the mouth. At an oxygen tension of 100 mm Hg this error could be as great as 8% if no correction is applied. Various methods for correction of this problem have been employed, but only to obtain a more accurate relationship between the gases of greatest interest. (See Scheid, P., Slama, H., and Piiper, J., "Electronic Compensation of the Effects of Water Vapor in Respiratory Mass Spectrometry," J. APPL. PHYSIOL., Volume 30, pages 258-260, 1971). Secondly, the sampling capillary introduces delay in response and deterioration of rise time of the instrument. Although this could theoretically be measured and corrected for, small variations in pumping speed cause relatively large changes in transit time such that in practice it is difficult to achieve this correction accurately. This creates problems when data concerning gas concentration are to be combined with other information such as gas flow rates as in the measurement of oxygen uptake. Lastly, even though the geometry of the sample conduit and inlet are fixed the actual rate of molecular flow into the spectrometer tends to vary from moment to moment because factors such as particle deposition and changes in gas composition alter the conductance of the inlet system.
Since the mass spectrometer is a particle counting device variations in molecular leak rate due to the above factors constitute a source of random error. Hence, it is apparent that the way by which the gas sample is introduced into the ionizer is the most critical step in the measurement of respiratory gases by a mass spectrometer. A more accurate measurement of the sample line would be made short and the volume of the conduits in front of the ionizer and the ionization chamber made smaller.